Various materials have been known to be used for active materials in nonaqueous-system secondary batteries. Among the materials, lithium composite metallic oxides, which have a lamellar rock-salt structure and are expressed by a general formula, LiaNibCocMndDeOf (where 0.2≤“a”≤1.5, “b”+“c”+“d”+“e”=1, 0≤“e”<1, “D” is at least one element selected from the group consisting of Fe, Cr, Cu, Zn, Ca, Mg, Zr, S, Si, Na, K, Al, Ti, P, Ga, Ge, V, Mo, Nb, W, La, Hf and Rf, and 1.7≤“f”≤2.1), have been used universally as active materials for lithium-ion secondary batteries.
However, when a lithium composite metallic oxide expressed by the aforementioned general formula is used as an active material in a high-capacity secondary battery driven or operated with a high voltage required for on-vehicle secondary battery, for instance, the lithium composite metallic oxide has been unable to keep the standard for satisfying a capacity maintained rate of the secondary battery, because the resistance of the material to the high voltage has been insufficient.
Consequently, investigations have been actively carried out recently to upgrade various materials to be used as active materials in the resistance to high voltage. In making the investigations, the following three methods have been proposed commonly.                1) doping an active material with an element of different species        2) forming a protective film on the surface of an active material        3) changing the composition of an active material in the superficial layer        
The method according to above-mentioned 1), and an advantageous effect thereof are concretely explained below. Doping an active material with an element, such as Al or Zr, which has not been present in the active material, enables degradations of the active material accompanied by charging and discharging operations, namely, accompanied by the absorption and release of Li, to be inhibitable.
The method according to above-mentioned 2), and an advantageous effect thereof are concretely explained below. As following Patent Application Publication No. 1 discloses, making a protective film on the surface of an active material with a salt of phosphoric acid, and preventing the active material from contacting directly with an electrolytic solution enable degradations of the active material resulting primarily from contacting with the electrolytic solution to be inhibitable.
The method according to above-mentioned 3) is concretely explained below. Following Patent Application Publication No. 2 discloses an active material with an increased Al composition in an obtainable superficial layer thereof by coating the active material on the surface with an Al compound and then heat treating the active material with the Al compound coated thereon.
Since the three methods according to aforementioned 1) through 3) have drawbacks given below, respectively, the methods have not necessarily arrived at obtaining a satisfiable active material yet.
The following are the drawbacks of the method according to aforementioned 1): Since absorbable and releasable Li in the active material has been decreased, in effect, by doping the active material with the different-species element not being driven or operated electrochemically, the Li storage capacity in the active material decreases and thereby the capacity of a lithium-ion secondary battery, per se, declines.
A drawback of the method according to aforementioned 2) is that the protective film formed on the surface of the active material turns into an electric resistance to make currents less likely to flow. Although making the protective film into an extremely-thin film is good to overcome the drawback, establishing such a technology is very difficult at the level of industrialization.
The method according to aforementioned 3) is desirable theoretically, because the method does not likely to cause a capacity to decline, the drawback of the method according to aforementioned 1), and because any electrically-resistive protective film, the drawback of the method according to aforementioned 2), is not formed at all. However, according to the disclosures of Patent Application Publication No. 2, since the method according to aforementioned 3) is virtually a technology of doping the active-material superficial layer with Al, not only the same drawbacks as 1) are observed, but also no marked advantageous effect is observed when comparing the active material of which the Al composition in the active-material superficial layer is increased by the treatment method set forth in the publication with another active material to which the treatment is not carried out at all.
That is, in the technologies of modifying the active materials, an active material fully satisfying the standard is not necessarily said to be obtainable.
Hence, the present inventors reported a novel lithium composite metallic oxide obtainable by a new surface modification method in Patent Application Publication No. 3.